In today's automatic transmissions, such as that shown in DE 198 58 541 A1, transmission stages are shifted by shift elements, i.e. clutches or brakes, formed as multi-disk shift elements or claw shift elements. The gearshift usually takes place hydraulically, through shift devices in the form of piston/cylinder units, which are subjected to pressure oil. The pressure oil is conveyed by a pump and is guided via oil supply lines from the transmission housing through so-called “rotary oil supply lines” into the rotating transmission shaft and then to the shift devices. Losses arise through the hydraulic circuit, such as through the performance of the transmission oil pump, the sealing points, a pressure drop in the supply lines or leakage. Losses burden the efficiency of the transmission. This invention is based on the fact that losses caused by the hydraulic circuit, in particular by the gearshift, must be kept as low as possible in order to achieve as high a degree of transmission efficiency as possible. A particular problem is that the oil pressure in a hydraulic cylinder with a closed shift element (the so-called “closing pressure”) must be constantly tracked (i.e., maintained) in order to transfer the necessary torque in the shift element. This pressure must be applied by the hydraulic pump, which is driven by the engine of the motor vehicle, by which losses are caused, such that the generation of hydraulic power increases fuel consumption and CO2 emissions, and reduces the available energy to drive the vehicle.
A hydraulically shiftable multi-disk shift element has been known from DE 102 05 411 A1 of the applicant, which, in a closed state (i.e. upon the transfer of torque) outside of gearshifts, is mechanically locked by a locking device. Thereby, the pressing force is maintained between the multi-disks without a hydraulic pressure and the transmission oil pump is unburdened. In one variant that is not shown, the locking device is formed as a stop valve, which is arranged in the feed area of the hydraulic cylinder. Thus, the pressure chamber of the cylinder to the pump is blocked and the contact pressure in the multi-disk pack is maintained.
Thus, the pressure to be generated by the pump may be lowered and the power input of the pump may thereby be reduced. The pressure generated by the pump is reduced to a necessary minimum and not completely switched off, since, even with closed and locked shift elements, the need for oil at a low pressure level continues to exist in the transmission, for example, for cooling, lubrication or pre-filling in preparation for a shifting process.
A pressure maintenance device of a transmission is known from U.S. Pat. No. 6,948,524 B1 which features a stop valve that encloses the actuating pressure of a shift element after the pump, acting as a pressure source, has discontinued its operation and the rest of the system is pressureless. The stop valve is formed in such a manner that, under the action of a spring force with a pump that is turned off (e.g. a pressureless hydraulic system), it moves into the shut-off position. Such a valve position is also referred to as “normally closed” and has the disadvantage that, with a pressureless transmission, all previously closed shift elements remain closed, by which the last transmission stage remains engaged.
In this manner, the functional reliability of the automatic transmission is disadvantageously affected, since, in special cases, such as upon engine standstill, the blocking of the automatic transmission may arise. The automatic transmission is also blocked, for example, against a tow movement.